A wet agitating ball mill of the friction grinding type is generally composed of a cylindrical enclosed stator, a rotor of the pin, disk or annular type disposed in the axial center of the stator to be rotated and driven by the motor, and the mill is filled with a media such as zirconia, glass beads, titanium oxide, steel balls or zirconia silicate, and the raw slurry in a material tank is supplied into the mill through a material pump, and the rotor is rotated and driven to agitate and mix the media and slurry, thereby grinding the slurry. The ground slurry is discharged out of the mill after separating the media by the separator, and is returned into the material tank. This operation is repeated and grinding is promoted. When reaching a desired product particle size, the mill is stopped, and the slurry in the material tank is transferred and collected in a product tank. After stopping the mill, the product slurry left over in the mill remains in the mill, but if it can solidify to disturb the next operations, the inside of the mill was cleaned by feeding cleaning water while operating the mill, and the slurry, diluted by the cleaning water and discharged from the mill, was discarded.
The media used in this type of mill were reduced in diameter as the product grain size became smaller according to the requests of customers and it was not rare to use media with a diameter of about 0.1 mm. One of the problems that must be solved in the mill for such fine pulverizing is the separation technology for separating the media efficiently from the slurry. As the separator for separating media from the slurry, hitherto, the screen and slit machines were used, but in the former screen type, it is extremely difficult to drill innumerable holes smaller than the media diameter, and if fabricated, the pressure loss is large, and clogging is likely to occur. In the latter slit mechanism, a representative example is composed of a disk fixed on a stator and a disk to be rotated fixed on the shaft and forming a slit which does not allow media to pass against the fixed disk by keeping a specific clearance to the fixed disk, and the media is separated in the slit between the disks, and the slurry is discharged through the slit, but it is extremely difficult to manufacture a slit width of about 0.1 mm, and if manufactured, the media are likely to be caught in the slit, and the disk is damaged easily. Additionally, since the slit width is narrow, there is limited slurry discharge amount, that is, grinding treating capacity of slurry.
As a separator capable of solving these problems, Japanese Laid-open Patent 4-61635 discloses a separator having two disks disposed parallel at a specific interval on the shaft, and coupling both disks by a spiral blade in an impeller form. This separator is designed to apply a centrifugal force to the media and slurry, and scatter the media having the greater specific gravity radially outward by making use of difference in specific gravity between the media and slurry, while discharging the slurry of the smaller specific gravity from a discharge route around the shaft and, therefore, since the same centrifugal force acts on the same diameter between the disks, the spacing of the disks can be widened and the treating capacity may be increased, and by widening the spacing of the disks, biting or clogging of the media in the disks can be avoided. Hence, the separation performance does not change with time and a stable operation is realized for a long period, and the media can be separated if the diameter is small, and micro media can be used, and fine pulverizing is hence possible, but in spite of these benefits, on the other hand, the slurry discharged from the discharge route around the shaft has a kinetic energy given by the rotation of the separator, that is, the action of centrifugal force, which means the kinetic energy is released wastefully, and unnecessary power is spent.
This separator is usually made of metal, but considering the contamination and wear by metal, it is preferred to use a ceramic. In the case of ceramics, it is extremely difficult to fabricate an integral body. Manufacturing the disks and blade separately, they can be assembled by adhering with an adhesive, but when the raw slurry comprises an organic solvent, the adhesive may fuse to have adverse effects on the quality of product slurry, or the separator may disassemble into individual parts.
Other problems occurring in the mill for fine pulverizing are the inaction of a mating ring or loss of function of a mechanical seal, in the mechanical seal as shown in FIG. 5 provided for shaft sealing of the shaft bearing portion, as the slurry and media get in and solidify in the clearance between the lower side of the fitting groove to which the O-ring is fitted and the mating ring.
It is a first object of the invention to present a method of recovering product slurry remaining in the mill after grinding, and it is a second object to present a grinding method capable of grinding efficiently by a mill.
It is a third object of the invention to decrease the power of operating a mill using the separator of the above impeller type, and it is a fourth object to avoid loss of function of the mechanical seal by preventing clogging by media and slurry that may impede the function of mating ring of mechanical seal. It is a fifth object to assemble easily the separator with disks and blades without using an adhesive.